| Summary: | The boron carbide (B<sub>4</sub>C) nanoparticles doping mesophase pitch (MP) was synthesized by the in-situ doping method with tetrahydrofuran solvent, and the corresponding MP−based carbon fibers (CFs) were successfully prepared through the melt−spinning, stabilization, carbonization and graphitization processes. The structural evolution and properties of boron−containing pitches and fibers in different processes were investigated for exploring the effect of B<sub>4</sub>C on mechanical, electrical and thermal properties of CFs. The results showed that the B<sub>4</sub>C was evenly dispersed in pitch fibers to provide active sites of oxygen, resulting in a homogeneous stabilization and ameliorating the split−ting microstructures of CFs. Moreover, the thermal conductivity of B1−MP−CF prepared with 1 wt.% B<sub>4</sub>C increased to 1051 W/m•K, which was much higher than that of B0−MP−CF prepared without B<sub>4</sub>C (659 W/m•K). While the tensile strength of B<sub>4</sub>C−doped CFs was lower than that of pristine CFs. In addition, a linear relationship equation between the graphite microcrystallite parameter (I<sub>D</sub>/I<sub>G</sub>) calculated from Raman spectra and the thermal conductivity (λ) calculated according to the electrical resistivity was found, which was beneficial to understand the thermal properties of CFs. Therefore, the doping B<sub>4</sub>C nanoparticles in MP did play a significant role in reducing the graphitization temperatures due to the boron catalytic graphitization but decreasing the mechanical properties due to the introduction of impurities.
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